These screens have significant advantages compared with liquid crystal displays (LCDs) as they emit light directly instead of modulating the transmission of light from a source outside the matrix. Therefore they do not need a light source. In addition, they have better contrast, they can be constructed on flexible substrates and they can provide images with excellent colorimetric qualities.
In certain cases, it is desirable to be able to display a given image with a variable average brightness without affecting the color rendition of the image. This is notably the case when it is desired that the screen can be watched comfortably in all kinds of outside light environment conditions. For example, in the sun, the screen must be very bright, otherwise nothing can be seen, and on the contrary, at night, the screen must not be dazzling to the observer, especially if they have to be able to look both at the screen and the outside night scenery. It is therefore desirable to provide for means of attenuating (‘dimming’) screen brightness in OLED screens, operable according to the circumstances and notably the outside light environment.
However, adjusting the overall brightness of the screen is not easy because of the characteristics specific to the light emission of OLED diodes. Adjusting the average brightness tends to change the colors of the image, which is to be avoided.
Organic light-emitting diodes are formed by the superposition of layers of organic semiconductor materials between two electrodes, a cathode and an anode, one of which is transparent or semi-transparent and the other is generally reflective in order to obtain an emission in one hemisphere. They emit light when traversed by a current and the greater the current is, the more intense the emission. The current in the diode and the voltage at the diode terminals are linked according to the specific characteristics of the diode. In general, the curve governing this relationship between current and voltage has the appearance shown in FIG. 1. To make it easier to understand, it can be said that they have an inactive zone or zone of high resistivity, for low voltages (less than 2 volts), in which the current is low and produces practically no light emission, then a useful zone of lower resistivity, in which the current increases sharply with the voltage (exponentially), and finally a saturation zone, for higher voltages, in which the current and the light emission increase further with the voltage but less fast than in the useful zone. Three curves are depicted in FIG. 1, for showing that the current, therefore the light emission, further varies substantially with temperature. In the curve example in FIG. 1, it is seen that the screen can benefit from a very wide current dynamic, therefore light emission, if a voltage varying between 2 and 4 or 5 volts is used.
Voltages and currents corresponding to the values of the useful zone are therefore applied individually to each pixel according to the image to be displayed. For this, an elementary circuit, associated with each diode, LED is provided at the intersection of each row and each column of the pixel matrix. This circuit can be used to select the pixel during a write phase for applying a control voltage to it corresponding to the desired light intensity. After the write phase the pixel retains the applied control voltage in memory and continues to emit the corresponding light intensity (except for leaks) up to a following write phase. A display in video mode or in parallel mode is possible. In video mode, all the pixels of a row are written successively then the pixels of the following row successively and so on. In parallel mode, the pixels of a row are written all at once, and then the pixels of the following row are written, and so on.
The basic constitution of a pixel of an OLED diode active matrix with its elementary circuit generally includes:                at least one control transistor having a source, a drain and a gate, capable of controlling the current flowing in the OLED light-emitting diode,        the light-emitting diode itself, having an anode and a cathode, one of the electrodes being connected to the source or to the drain of the control transistor, the other electrode being common to a plurality of pixels in the matrix,        means for driving the control transistor according to the information to be displayed by the pixel.        
Various configurations are possible, the control transistor notably being able to be of the NMOS or PMOS type, and the electrode common to a plurality of pixels being able to be connected between the control transistor and a low power supply potential or between the control transistor and a high power supply potential.
FIG. 2 depicts an example of pixel configuration of an organic diode active matrix. The pixel includes:                the OLED light-emitting diode corresponding to this pixel, of which the cathode is connected to a cathode potential Vk,        an NMOS control transistor Qc of which the source is connected to the anode of the OLED diode and of which the drain is connected to a power supply voltage source Vdd which can supply the current necessary for light emission;        a selection transistor Qs that is used to enable the application of a gate voltage Vdat to the gate of the control transistor; this voltage Vdat is an analog voltage whose value varies according to the light emission desired for the pixel; it is applied to the drain of the transistor Qs by a column driver Cj common to all the pixels of a same column of rank j of the matrix; the column driver receives and transmits a voltage Vdat for a given pixel when this pixel is selected by the selection transistor Qs; the source of the selection transistor Qs is connected to the gate of the control transistor Qc; the gate of the selection transistor Qs is connected to a row driver Li common to all the pixels of a same row of rank i of the matrix;        a storage capacitor Cst connected between the drain and the gate of the control transistor; this capacitor maintains the voltage applied to the gate of the transistor Qc throughout an image frame, after a voltage Vdat has been applied to this gate at the time the pixel is written.        
The storage capacitor is not always needed, notably if the parasitic capacitance of the transistor (between gate and source-drain) is sufficiently high to be able fulfill this role of maintaining the voltage for the duration of a frame.
The operation of a matrix using this elementary pixel circuit is as follows: the pixels of the first row are written by making the selection transistors of this row conductors; then, in video mode, the individual voltages Vdat to be applied to the successive pixels of the row are applied successively to the various columns of the matrix; in parallel mode, the voltages would be applied simultaneously on all the columns; in both cases, the voltage Vdat assigned to one pixel is transferred over to the gate of the pixel's control transistor and to the associated storage capacitor Cst, which generates a light emission; the light intensity depends on the voltage Vdat, since this controls the flow of current in the transistor and in the OLED diode. After writing in a pixel, the storage capacitor Cst maintains the potential Vdat on the gate, up to a following write phase. Accordingly, the pixel maintains the light emission corresponding to this voltage Vdat until the following write, i.e. for the duration of an image frame.
An image frame includes the successive writing of all the pixels of all the rows of the matrix. In addition, in video mode, there are idle times (‘row blanking’) at the beginning and end of writing each row, and at the beginning and end of writing each frame (‘frame blanking’).
It will be understood that if a same image is to be displayed very brightly (for daytime ambient conditions) or with low brightness (for nighttime ambient conditions), all the voltages Vdat can be modified for adapting the image to the ambient conditions and displaying darker images in the second case thanks to much lower voltages. But first this requires an extension of the input dynamic over several decades and secondly, given the highly non-linear form of the emission characteristics of OLEDs (FIG. 1), it is very difficult to maintain the same image quality for both cases, in particular in terms of color retention, especially if it must be done for a plurality of levels of mean luminance.
The brightness of the screen may also be modified by acting on the value of the cathode voltage Vk without modifying the analog voltages Vdat representing the image and without modifying the voltage Vdd of the power supply source: raising Vk clearly means that there is an overall downward movement of the characteristic in FIG. 1. But here again, as the foot of the curve is approached, the more the color characteristics of the image change.
Patent publication US2006/0164345 further proposed a pixel circuit scheme tending to apply the voltage Vk to the cathode of the OLED diode for a part of a cycle and to interrupt this application for the rest of the time. An attenuation transistor, alternately turned on and blocked by variable duty cycle pulses (“Pulse Width Modulation” PWM) on its gate, is placed in series between the cathode of the diode and the cathode reference at the potential Vk. According to the switching duty cycle, the average brightness of the screen can be varied without modifying the voltage Vdat pattern to be applied to the matrix.
This scheme and other schemes of this publication therefore act through temporary interruption of the current in the OLED diode, by removing the negative power supply or the positive power supply for a variable duration.
However, when the negative voltage Vk ceases to be applied, it is found that the current in the OLED diode is not interrupted immediately as would be desired. This results from parasitic capacitances that impede the instantaneous removal of the voltage present at the diode terminals. The current present in the LED while the negative power supply Vk is applied tends to persist for some time, notably because the capacitance existing naturally between the electrodes of the diode maintains a voltage at the terminals thereof; this capacitance is gradually discharged due to the current flowing through the diode, and the current is gradually reduced, gradually reducing the emission of light. This reduction depends largely on the current that exists in the diode just before switching. It therefore varies from pixel to pixel. Because of this, the resulting average reduction in intensity of emission in one pixel for a given duty cycle, therefore depends on the initial state of the pixel. It does not lead to a uniform reduction in brightness and the image is distorted, notably in terms of colorimetry, when it is wished to attenuate its average brightness.
It may be added that for picture elements of low brightness, the discharge of the voltage at the diode terminals is particularly slow when the power supply via Vk is interrupted, so that for low duty cycles there may, in fact, be no brightness reduction for these pixels.
It was further proposed in patent application EP1 061 497 to reduce brightness by acting on the cathode voltage, but the device described does not enable average attenuations to be established, or requires that the cathodes of the OLEDs are grouped by rows independent of the cathodes of the other rows.